Carbohydrates have increased in importance as bio-based starting materials for a wide range of chemicals. One development is the conversion of carbohydrates to 5-hydroxymethyl furfural and ethers and esters thereof. A process for the conversion to an ether derivative has been described in WO 2007/104514. According to this process a fructose and/or glucose-containing starting material is converted to an ether derivative of 5-hydroxymethylfurfural by reacting the starting material with an alcohol in the presence of an acid catalyst. WO 2012/091570 describes that in order to increase the concentration of the starting material in the feed stream of the process it is beneficial to convert the carbohydrates to a glycoside before being converted to the ether. From this document it is also apparent that the solubility of glucose is lower than that of fructose in the reactant medium, which includes an alcohol.
It would therefore be advantageous if it were possible to provide a process with a high level of fructose that could be used in the process for the production of derivatives of 5-hydroxymethylfurfural.
The process for the provision of a fructose-rich product is known in the prior art. In US 2010/0006091 a process is disclosed wherein a sweet fruit juice is clarified and demineralized and the product thus obtained is then processed to hydrolyse sucrose into fructose and glucose. The fructose is separated from the glucose. The glucose is subsequently isomerized to fructose, and the two fructose fractions are combined. According to the document the isomerization of glucose is achieved using an enzyme with glucose isomerase activity. In this known process the isomerization is conducted in an aqueous environment. This results in a product that becomes available in a significant amount of water. However, if the fructose produced is to be used in a non-aqueous environment the water content becomes a drawback, as the water is then to be removed. Removal of this water can be accomplished by evaporation, but such evaporation adds considerably to costs. Therefore, the process for the production of ethers of 5-hydroxymethylfurfural as described in WO 2007/104514 would benefit if the starting materials would be available in an alcoholic medium. Since the activity of many enzymes is negatively affected in alcoholic mediums, the process according to US 2010/0006091 is not suitable for the intended purpose.
In U.S. Pat. No. 3,431,253 the isomerization of glucose to fructose over an alumina catalyst has been described. The isomerization is accomplished by contacting a solution of glucose with alumina at temperatures from about 35 to 70° C. At temperatures above 70° C. it is stated that the formation of organic acids occur. Although it is stated in the description of U.S. Pat. No. 3,431,253 that it is possible to use mixed water-lower alcohol solutions, such as 90% methanol, 80% ethanol and 75% propanol solutions, in the examples only 100% aqueous solutions of glucose are contacted with alumina at 50° C.
A similar procedure is described in a journal article by H. Asaoka, Carbohydrate Research 137 (1985) 99-109, wherein the isomerization of glucose to fructose was accomplished in a 80% (v/v) methanol-water solution over a disodium pentasilicate catalyst at 45° C. When ethanol or 1,4-dioxane was used instead of methanol, the results were worse. All attempts with propanol, acetone, acetonitrile and tetrahydrofuran gave unsatisfactory results.
Similar teachings are described in Lew et al., Ind. Eng. Chem. Res., 51 (14) (2012) 5364-5366, disclosing the conversion of glucose to fructose over Sn-containing Lewis acid zeolite beta in ethanol at 90° C. The reaction is conducted in the presence of ion exchange resin Amberlyst 131 which catalyses the reaction of fructose formed to hydroxymethyl furfurural and subsequently to ethoxymethyl furfural. In Chem. Abstr., 2011: 336348 by Canlas et al. the conversion of glucose to fructose over Ti- or Sn-containing Bronsted or Lewis acid zeolites in water or methanol at 100 to 160° C. is described. It is observed that the above-mentioned prior art documents do not relate to the formation of fructosides.
In Saravanamurugan et al., J. Amer. Chem. Soc., 135 (14) (2013) 5246-5249 the isomerization of glucose in methanol over acidic zeolites has been described. The product obtained is stated to yield fructosides, which are subsequently hydrolyzed to fructose. In the article it is shown that basic zeolites yield considerably lower amounts of fructose. Whereas acidic catalysts yield more than 20% fructose, the basic catalysts yield from 4 to 18% fructose.
Thus many documents in the prior art stipulate that the isomerization reaction medium contains water. In contrast therewith, it has now been found that it is not necessary that water is present if the reaction temperature is above 75° C. The solvent may then consist of pure alcohol. If water is absent it has been found that no undue formation of organic acids occurs. It has further been found that when the isomerization reaction is conducted in the presence of a basic catalyst and the fructoside formation is conducted in the presence of an acidic catalyst, the amount of fructoside is enhanced.